Richtig, denke ich auch mit dem Komma. Aber heute lautet es ja: im Zweifel weglassen.

Grammatik hingegen ist definitiv weiblich.

"den" is a pronoun which does not make sense/does not fit the gender in this context.

So I changed it to "denn" which is an adverb which in this case is hard to translate but boils down to something like "at all" in this sentence. Eg do you learn grammar there at all.

If you want to use a pronoun with the correct gender then it would be "die" (female) which is technically correct but sounds weird in this case.

So you could simply leave it out which would sound more natural.